1. Field of the Invention
The present invention relates to a technique for high-linearization of variable gain amplifiers.
2. Related Art
At a receiver receiving a radio signal, since the power of a radio signal is around 100 dBs, the amplitude of the received signal must be controlled to a predetermined level at an input of an A/D converter that converts a received analog signal into a digital signal.
On the other hand, once the receiving side realizes that excessive input power is being sent, a transmitter also controls transmitting power so that a predetermined level may be received by the receiving side.
Functions for arranging receiver gain or transmitting power to be variable are performed at a variable gain amplifier. A conventional example of a variable gain amplifier VGA is shown in FIG. 13(A). A variable current source I1 is connected between common source terminals of MOS transistors M1 and M2, which form a differential pair SP1, and a ground. Output current is retrieved from drain terminals of the MOS transistors M1 and M2. For simplification of illustration, the differential pair SP1 is shown as in FIG. 13 (B).
Variable gain differs depending on operation regions of the MOS transistors M1 and M2. An operation region in which a gate-source voltage is higher than a threshold voltage and a drain current increases in proportion to square of the gate-source voltage is referred to as a strong inversion region, while an operation region in which a gate-source voltage is lower than a threshold voltage and thus hardly any drain current flows is referred to as a weak inversion region. In the relationship between a gate-source voltage and a drain current, the strong inversion region exhibits a square-law characteristic while the weak inversion region exhibits an exponential characteristic.
When the MOS transistors M1 and M2 have square-law characteristics, a transconductance gm corresponding to gain is given bygm=2√(βId),where β=(½)μCox(W/L), μ is mobility, Cox is oxide film capacitance, “W” is gate width and “L” is gate length. Id represents the drain current.
When the MOS transistors M1 and M2 have exponential characteristics, the transconductance gm is given bygm=Id/(nVT),where “n” is a constant related to processes, and VT is thermal voltage, which is 26 mV at room temperature.
An input range of an input voltage Vin, which is formed of a potential difference between voltages respectively applied to gate terminals of the MOS transistors M1 and M2, will now be examined in a case in which the MOS transistors M1 and M2 have square-law characteristics. In this description, for instance, a range in which linearity is maintained (a range in which gain is constant) with small error is used as an input range “R”. However, the input range “R” may include a nonlinear range in which linearity is not maintained (a range in which gain varies according to changes in the input voltage Vin).
When the MOS transistor M1 is cut off (no current flow) and a current Id only flows through the MOS transistor M2, a voltage Vlim across the gate terminals of the MOS transistors M1 and M2 is given byVlim=√(Id/β).The voltage Vlim will act as an indicator for the input range “R” of the variable gain amplifier (differential amplifier) VGA.
In the above equation expressing the voltage Vlim, decreasing the current Id in order to lower gain will result in a decrease of Vlim. FIG. 14 shows the voltage Vlim at a transconductance G1max when current Id is maximized to maximize gain and at a transconductance G1min when current Id is decreased to minimize gain.
As seen, the lower the gain, the greater the deterioration of the linearity of the variable gain amplifier VGA (in other words, the smaller the input range “R”). However, in this case, it is assumed that the effect of distortion at the output-side of the variable gain amplifier VGA is not considered. By further decreasing the current Id, the square-law characteristic changes to an exponential characteristic. Since the exponential characteristic is almost the same as a characteristic of a bipolar transistor, the input range “R” will decrease to around nVT. Beyond this point, the input range “R” will no longer decrease.
A transmitting section TX using a direct modulation method is shown in FIG. 15. A quadrature modulator OM having mixers MX1 and MX2 and a phase shifter PH modulates an LO signal outputted from a local oscillator LO11 using an I/Q signal obtained sequentially via a D/A converter and an LPF. An output thereof is inputted to the variable gain amplifier VGA, where signal amplitude of the output is adjusted using a control signal. An output of the variable gain amplifier VGA is then amplified by a next-stage power amplifier PA, and radio waves are emitted from an antenna, not shown, via a filter BPF. In this case, the output of the quadrature modulator OM remains constant regardless of power emitted from the antenna.
When using the above-described variable gain amplifier VGA, since lowering gain results in a decrease of the input range “R”, the output signal will be distorted. Since distortion of the output signal causes deterioration of modulation accuracy or increase in unwanted radiation, it is necessary to minimize distortion in output signals.
[Patent Document 1] Japanese Patent Laid-Open No. 2001-196880
[Non-Patent Document 1] T. Yamaji, et. al., “A temperature-stable CMOS variable-gain amplifier with 80-dB linearly controlled gain range”, IEEE J. Solid-State Circuits, pp. 553-558, May, 2002